Electrochemical reactions form the basis of many important commercial applications. For example, batteries and fuel cells utilize electrochemical reactions to convert the dormant energy stored in chemical reactants into electricity. Additionally, several large-scale synthetic processes involve electrochemical reactions including the electrolysis of salts or solutions to produce elemental forms of active materials such as aluminum, lithium and sodium. In each of these types of applications, the performance and thus the value of the device or process is limited by the materials used. In particular, it is highly desirable that the application or process be highly efficient, thus maximizing energy conversion in the case of batteries and fuel cells and minimizing energy costs in the case of electrolytic processes.
Applications and processes that have poor electrochemical efficiency suffer losses of much of the available or supplied energy in the form of heat generation according to the equation ΔH=PΔt=i2RΔt, where R is the effective resistance of the cell, i is the current density, P is the power loss and Δt is time. Thus more energy is lost to heat generation when operating an inefficient electrochemical process relative to a more efficient process. In the case of a battery or fuel cell, a more efficient device will exhibit greater power density and greater energy density, particularly when the power demand is high. In fact, much of the design and cost of a battery or fuel cell system, particularly for large, high-power systems used in applications such as electric and hybrid electric or fuel cell vehicles, involves the minimization and management of heat generated by the system during operation.
The efficiency of an electrochemical process or device is dependent on many factors. These factors include the design of the electrochemical cell, the materials used to make the cell, the kinetics of the reactions occurring in the cell, and the multiple interactions of the various materials comprising the cell. To obtain a truly accurate measure of the performance potential of a specific electrochemical material composition, it is critical that all of these issues and interactions are part of the testing environment. For example in a Li-ion battery, the efficiency of the battery can be affected by a number of factors including the kinetics of the intercalation reaction at both the anode and cathode, the electrical conductivity of the anode and cathode, the porosity of the anode and cathode electrodes, the conductivity of the electrolyte, or the porosity of the separator among other factors. In a fuel cell, the efficiency of energy conversion can be greatly affected by the catalyst over-potential, which must be minimized, electrode composition and fuel distribution.
Thus, it is highly desirable to evaluate new electrochemical material candidates in a conventional cell that provides a testing environment similar to that for which the material is intended. This can be particularly important for systems in which interactions between the anode and cathode chemistry can affect the material performance. Such phenomena are common in both battery and fuel cell systems. For a battery or fuel cell a conventional cell commonly comprises a membrane tightly sandwiched between two electrodes; an anode at which oxidation occurs, and a cathode at which reduction occurs. An electrolyte for ion conduction is shared by the anode and cathode.
Because of the importance of the testing environment most electrochemical materials development is still done in series, where individual cells are made for each material and evaluated utilizing a single electronic load or cycler channel for each cell. Traditional current-voltage methods are employed to probe the performance of the materials over long periods of time, requiring large numbers of cycler channels, electronic loads and monitoring equipment. For example, one of the key performance criteria for hybrid electric vehicle batteries and fuel cells is that the there be little change in the resistance or efficiency of the device over the 10-15 year life of the application. Such long-term performance requirements make serial development of materials for such applications extremely difficult and costly since in many cases a single channel could potentially be occupied for months if not years simply to evaluate one material composition or cell design.
Predictive calculation and modeling of the performance of new electrochemical materials could mitigate some of the development burden. Unfortunately, many interfacial electro-catalytic reactions, such as those on which a hydrogen fuel cell is based, are very complex and not readily predisposed to rational catalyst design and many of the factors that affect the life of a battery or fuel cell are not well understood and thus difficult to accurately model. As a result, it can be a very time consuming process to discover and optimize new, more efficient electrochemical material compositions by conventional methods. A combinatorial approach to materials discovery, in which many compositions can be evaluated simultaneously and accurately, can be greatly beneficial to this process, and can be very valuable to the battery, fuel cell and electrolytic industries.
A number of methods have already been developed to screen various electrochemical materials combinatorially. Most of these methods involve the creation of arrays of electrodes or electrochemical cells on a single substrate, each individually addressable by an isolated electrical connection. Examples include U.S. Pat. No. 6,187,164 and US Published Patent Application Nos. 2002/028456 and 2003/0070917. While semiconductor processing methods have allowed large arrays to be made on very small substrates, testing of the materials still require a large number of electrochemical testing channels to probe each electrode by conventional voltage-current techniques. Furthermore, the electrode array structures generally do not allow for the design of electrochemical testing conditions that accurately simulate the environment in which the material will be utilized. For example, the members of the electrode array are commonly tested under half-cell conditions in a flooded cell environment. Semiconductor processing methods have also been used to make similar material arrays for testing a variety of non-electrochemical processes. For example, thermal imaging of sputter deposited alloys has been used as a probe of conventional catalytic reactions, as disclosed in published international application WO 99/34206, and of phase changes of materials, as disclosed in U.S. Pat. No. 6,536,944.
A conventional fuel cell device has been developed that can test multiple fuel cell catalysts in parallel against a common electrode to ensure more accurate comparison and evaluation of the catalyst samples. This device also uses conventional voltage and current techniques to probe performance and requires individual current monitoring channels for each electrochemical sample, as disclosed in US Published Patent Application Nos. 2002/0009627 and 2004/0224204. A highly parallel indirect screening method has been developed, also primarily for fuel cell catalysts. The method and devices using the method rely on indicator molecules to provide an optical signal whose intensity is related to the extent of the reaction of interest, as disclosed in published international applications WO 2000/04362 and WO 2002/05367. As an indirect method, a single voltage source can be used to power the device and simultaneously probe a large number of catalyst samples. However, a clear line of vision of the reaction front is required, preventing the use of conventional cell designs and diminishing the accuracy of the screening method. Furthermore, there are also many electrochemical processes for which suitable indicator molecules have not been identified.
Despite these advances, a combinatorial method for screening a large number of electrochemical materials samples over long periods of time and in conventional cell environments at a reasonable cost is needed.